“Persistent mismanagement” of the city’s overhaul of the 911 system has left the project incomplete, resulted in years of delays and added hundreds of millions of dollars to its cost, according to a report by the city’s Department of Investigation.

The overhaul was started in 2004 by then-Mayor Michael R. Bloomberg after troubles with the system during the Sept. 11, 2001, terrorist attacks and the widespread blackout of 2003.